Such bandgap reference voltage generator circuits serving to generate a reference voltage which is practically independent of temperature for a (especially as compared to Zener diodes) relatively low supply voltage are based on the fact that with increasing temperature the base/emitter voltage of a bipolar transistor falls, whilst the difference in the base/emitter voltages of two bipolar transistors, whose current densities relate to each other in a fixed predefined ratio, increases with rising temperature. When the sum of these two voltages, depending on the temperature in opposite directions, corresponds to the bandgap of the semiconductor, e.g. around 1.205 V for silicon, it represents a reference voltage which is practically independent of temperature. This is why these circuits are also simply termed bandgap references.
A bandgap reference voltage generator circuit of the aforementioned kind is described e.g. by A. Paul Brokaw in the paper xe2x80x9cA Simple Three-Terminal IC Bandgap Referencexe2x80x9d in IEEE Journal of Solid-State Circuits, Vol. SC-9, No. 6, December 1974.
In the bandgap reference voltage generator circuit as shown in FIG. 2 representing that of the cited paper and which is shown in FIG. 1 as described in the present invention, the control means consist of an operational amplifier, the one input of which is connected to the collector of the first transistor whilst its other input is connected to the collector of the second transistor and whose output is connected to the bases of both transistors.
In a further embodiment of the bandgap reference as shown in FIG. 3 of the cited paper the control means consist of a current mirror, the one current branch of which is connected to the collector of the first transistor and whose other current branch is connected to the collector of the second transistor, and a further transistor whose one control input is connected to the collector of the second transistor and whose current path is connected to the bases of both transistors. One such circuit is also shown in FIG. 2 of the present description.
In the two embodiments as shown in FIGS. 2 and 3 of the cited paper and FIGS. 1 and 2 of the present description the first resistor is connected between the two emitters of the two bipolar transistors and is, in addition, located in the collector current path of the first bipolar transistor.
In the two prior art band pass references fabricated as a rule integrated, complicated tuning procedures are needed, as a rule, to compensate the production errors and tolerances of the components employed; it often being the case, namely, that fabricating the integrated circuit results in a mismatch between the circuit components employed. This may be e.g. a mismatch between the two current mirror transistors in the embodiment as shown in FIG. 3 (or in FIG. 2 of the Figures belonging to the present description) of the aforementioned paper. In the embodiment as shown in FIG. 2 of the aforementioned paper offsets of the input currents of the operational amplifier may occur caused by errors and tolerances in the components of the operational amplifier. It will readily be appreciated that such faults are particularly serious in a circuit whose task it is to generate a reference voltage for other circuits and are capable of substantially decrementing proper functioning of the circuit.
It is thus the objective of the invention to provide a bandgap reference voltage generator circuit of the aforementioned kind which is more immune to production errors in the components and in which the tuning procedures employed hitherto for correcting component errors are now simplified or even eliminated.
This objective is achieved by a bandgap reference voltage generator circuit of the aforementioned kind in which the first resistor is connected between the base terminals of the two transistors and is, in addition, connected to the collector of the second transistor.